str.Split('&')
.Select(s=>s.Split('=')
.Skip(1)
.FirstOrDefault()).ToArray();
OR
str.Split(new[] { "section[]=" }, StringSplitOptions.RemoveEmptyEntries)
.Select(s => s.Replace("&", ""))
.Select(Int32.Parse).ToArray();
OR
var items = new List<string>();
foreach (Match item in Regex.Matches(str, @"section\[\]=(\d+)"))
items.Add(item.Groups[1].Value);